An optical fiber used in an optical fiber communication system that is currently popular has a structure in which the periphery of one core is coated by a clad to allow optical signals to propagate through the core so as to transmit information. In recent years, an amount of information significantly increases along with popularization of optical fiber communication systems. To handle such an increase of the amount of information to be transmitted, a large number of, that is, tens to hundreds of optical fibers are used in an optical fiber communication system so as to perform large amount and long distance optical communications.
To decrease the number of optical fibers in such an optical fiber communication system, it is known to use a multi-core fiber in which the peripheries of a plurality of cores are coated by one clad to allow lights to propagate through the respective cores so as to transmit a plurality of signals.
As such multi-core fibers, a non-coupled multi-core fiber and a coupled multi-core fiber are known. In a non-coupled multi-core fiber, respective cores work as transmission passes independent of each other and the cores are coupled as weakly as possible. In a coupled multi-core fiber, respective cores are coupled to each other so that the plurality of cores can be substantially regarded as one multimode transmission path. This coupled multi-core fiber enables mode multiplexing transmission which transmits different signals for respective modes of lights propagating through the cores.
Non Patent Document 1 listed below discloses an example of such a coupled multi-core fiber. According to Non Patent Document 1, the closer the respective cores of a coupled multi-core fiber are arranged, the stronger the cores are coupled. Therefore, it can be thought that the cores are coupled most strongly when the cores contact with each other.    [Non Patent Document 1] Yasuo Kokubun “Novel multi-core fibers for mode division multiplexing: proposal and design principle” IEICE Electronics Express, Vol. 6, No. 8